Focused ion beam systems direct charged particles onto a work piece, or target, for processing the work piece or for forming an image of the work piece. Charged particle beam systems are used, for example, in integrated circuit fabrication and other nanotechnology processing. Charged particles beam systems typically include a source of particles, a beam blanker, accelerating lenses, focusing optics, and deflection optics.
High resolution focused ion beams (FIBs) have proven useful for a variety of tasks such as microscopy, lithography, micromachining (ion milling and material deposition), and dopant implantation. Over the years, a number of ion sources have been developed for focused ion beam applications, including gas-phase field ionization, plasma, and liquid metals. Of all the sources developed to date, the liquid metal ion source (LMIS) has proven the most useful and is in the most widespread use today. The usefulness of the liquid metal ion source stems fundamentally from its very high brightness which allows the production of focused ion beams with spot sizes on the order of 10 nm while maintaining currents in the range of 1 pA to 10 pA. These characteristics give focused ion beams the necessary resolution and ion currents to perform a range of state of the art nanotechnology tasks.
Despite their widespread use, existing ion sources possess limitations that impede progress toward broader applications and higher resolution. The use of focused ion beams with high landing energies at the target, which is above 5 keV, can cause significant damage to the work piece. However, the use of beams with low landing energy results in poor spot size performances needed to make thin lamellas.
Accordingly, a need exists for an improved system and strategy for a focused ion beam with low keV landing energy but effective spot sizes. Focused ion beams with improvements in low keV can be achieved with low C objective lens or tetrode/pentode switchable lenses.
The higher the immersion ratio, the stronger the cathode lens becomes. Axial aberration coefficients drop significantly with increasing immersion ratio k. For example, the coefficient of chromatic aberration C, which has a major impact on resolution at low landing energies, is almost inversely proportional to the immersion ratio k. This leads to partial compensation for the beam diameter deterioration at low beam energy seen in the cases where beam deceleration is not used. But the performance improvements in using low C objective lenses and tetrode/pentode switchable lenses is very small and not sufficient for imaging and creating ultrathin lenses.
Furthermore, it is difficult to accelerate ions from a source without inducing a large energy spread in the resultant beam. The larger the spatial extent of the ion source, the more difficult it is to focus the ions to a point. Improvements in the system are required to produce smaller probe sizes and produce the resolution that such systems are theoretically capable of producing.
The focusing optics focus the beam into a spot or a predefined shape on the surface of a target. Focusing optics typically include a combination of condenser lenses and an objective lens. The lens can be electrostatic, magnetic, or various combinations of the two. Charged particle lenses, like light lenses, have aberrations that prevent the particles from being focused to a sharp image. The aberration is least for charged particles passing through the center of the lens, and the aberration increases as the distance from the center of the lens increases. It is desirable, therefore, for the charged particle beam to pass very near the center of the lens. One type of aberration, referred to as “beam interaction” occurs because the particles in the beam, all having the same electrical charge, repel each other. The closer the particles are to each other, the greater the repulsive force. Because the particles are typically converging after passing through the objective lens, it is desirable to position the objective lens as close as possible to the work piece, to reduce the time that the particles are focused in a tight beam. The distance between the objective lens and the work piece is referred to as the “working distance.”
The deflection optics direct the beam to points, referred to as “dwell points” or “pixels,” on the surface of the work piece. For example, the beam may be directed in a raster pattern, in a serpentine pattern, or toward an arbitrary sequence of individual points. The beam will typically dwell at a point for a specified period, referred to as “dwell period,” to deliver a specified “dose” of charged particles, and then be deflected to the next dwell point. The duration of the dwell period is referred to as the “dwell time” or the “pixel rate.” (While pixel “rate” more properly refers to the number of pixels scanned per second, the term is also used to indicate the time the beam remains at each pixel.)
The deflection optics can be magnetic or electrostatic. In focused ion beam systems, the deflection optics are typically electrostatic. Electrostatic deflectors for focused ion beam are typically octupoles, that is, each deflector includes eight plates, distributed around the circumference of a circle. Different voltages are applied to the eight plates to deflect the beam away from the optical axis in different directions.
If the deflector is placed below the objective lens, the beam can pass through the center of the objective lens to minimize aberration. Such a configuration is used, for example, in the VisION System sold by FEI Company, the assignee of the present invention. Placing the deflector below the objective lens, however, increases the working distance, thereby increasing the beam aberration.
To minimize the working distance, one can place the deflector above the objective lens. With the deflector above the lens, however, when the beam is deflected, it is moved away from the center of the lens, thereby increasing certain aberrations. To solve this problem, many focused ion beam systems use a two stage deflectors.
What is needed is a focused ion beam that can achieve the necessary optimum spot sizes to work on ultrathin lamellas without the high landing energy at the target.